1. Field of the Invention
The present invention generally relates to a power inverting method and a system capable of parallel-connecting different types of a.c. power supplies having different capacities to a common bus line. More specifically, the present invention is directed to a power inverting method and system wherein a plurality of uninterruptible power supplies are independently connected in parallel with the common bus line (load).
2. Description of the Prior Art
As highly reliable power supplies used for a computer system or the like, a so-called "Uninterruptible Power Supply (simply referred to as an UPS)" has been widely utilized. The UPS is capable of stably supplying AC power to such a specific load, e.g., a computer system, by employing a battery unit even under such difficult conditions that a supply of AC power from an AC power source is interrupted, or an AC output voltage of the AC power source is instantaneously lowered. In particular, very recently, a large-scaled power supply system for driving a plurality of UPS devices in a parallel mode is required in order to accept demands to increase power capacities and also to improve power supply reliabilities, required by a large-scaled electronic system such as an on-line communication network.
A parallel driving technique for a plurality of UPS, or power inverting units is known in the art where either a shortage of power capacity in a bus line happens to occur, or reliabilities on the power supply from the bus line to the load are increased.
Such a parallel driving technique of the plural power inverters has been described in, for instance, Japanese patent publication No. 58-46955 published on Oct. 19, 1983, and granted under number of 1215332.
However, when the outputs of the power inverting units (e.g., power inverters) are connected to the bus line, and driven in the parallel mode so as to increase the total power supply capacity of the bus line to the loads, the above-described conventional power supply systems have the following problems:
(1) Among the respective power inverting units, the control signals are necessarily required to be mutually communicated. This implies that these power inverting units cannot be independently driven, or controlled. As a result, the troubles such as malfunction of one power inverting unit may adversely influence operations of the other power inverting units. Then, even when "N" power inverting units ("N" being an integer, more than 2) are driven in the parallel mode on the single bus line, the total power capacity of this bus line cannot become "N" times greater than a total power capacity of a single power inverting unit, but may become smaller than "N" times. Furthermore, higher power-supply reliability of the parallel-driven power inverting system cannot be expected.
(2) Since each of these power inverting units is controlled in such a manner that both active power and reactive power must be equally outputted therefrom, in principle, only power inverting units having identical rated voltage/current and power-supply characteristics can be driven in the parallel mode.
(3) When one or more power inverting units are additionally employed in the present parallel-driven power inverting system in order to solve a shortage of output power of this power inverting system, due to a heavy load, the following problems may additionally occur:
3A) Since there is a limitation in the various conditions for the parallel-driven power inverting units as described in the above item (2), it is originally required to establish a design policy capable of accepting an expansion of power supply capability. However, nowadays, it is practically very difficult to establish such a future power-supply scheme, taking account of drastic changes in the technology. Eventually, it will be impossible without the provision of a separate power inverting system.
3B) Even when one or more power inverting units can be additionally provided with the original bus line, the control signals of the additional power inverting units must be communicated with the presently driven power inverting units, because of the restriction item (1). There are other limitations in time periods and timings of installation constructions.
The present invention has been made in an attempt to solve the above-described problems, and therefore has a primary object to provide such a flexible power inverting system that no control signal is mutually communicated among a plurality of power inverting units driven in a parallel mode, and also no restriction is required in characteristics/rating data of these power inverting units.
A secondary object of the present invention is to provide a power inverting system capable of driving plural power inverting units in a parallel mode without any limitations in power capacities or voltage waveforms of a bus line.